Circuit emulation maintaining transport overhead integrity

ABSTRACT

Techniques for emulating a time division multiplexed (TDM) circuit using a packet switched network are described. First and second packet nodes are installed in a communication network. The first TDM node and second TDM node form a first span in the TDM circuit. First and second fiber connections are disconnected from the first and second TDM nodes, and connected to the first and second packet nodes. A portion of TDM data transmission across the first span is emulated using a packet connection formed by the first packet node and the second packet node. The TDM data transmission includes transport overhead data, and the packet connection emulates the transport overhead data.

TECHNICAL FIELD

Aspects of the present disclosure relate to communication networks, andmore specifically, though not exclusively, to circuit emulationtechniques for time division multiplexed networks.

BACKGROUND

Circuit emulation technology is becoming popular for network operatorsas a way to migrate away from legacy synchronous optical networking(SONET) and synchronous digital hierarchy (SDH) networks to packetswitched networks. For example, network operators can emulate circuitswitched time-division multiplexing (TDM) networks, like SONET and SDHnetworks, using various packetization techniques. These techniques caninclude structure-agnostic time division multiplexing over packet(SAToP), structure-aware time division multiplexed circuit emulationservice over packet switched network (CESoPSN), and circuit emulationover packet (CEP).

One challenge for network operators, however, is completing thismigration efficiently and effectively. It is often not possible toconvert a SONET or SDH network all at once to a packet switched network.Instead, new packet resources are rolled out over time. Further, anetwork operator may wish to migrate part, but not all, of a circuitswitched network. One or more embodiments described herein relate totechniques to allow migration of various TDM services, one by one, whilekeeping operation of the legacy SONET or SDH network infrastructureintact.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this disclosure and are therefore not to beconsidered limiting of its scope, for the disclosure may admit to otherequally effective embodiments.

FIGS. 1A-B illustrate a legacy synchronous optical networking networkand a corresponding migrated packet network, according to one embodimentdescribed herein.

FIG. 2 illustrates a circuit emulation controller, according to oneembodiment described herein.

FIGS. 3A-C illustrate packetization of a SONET frame, according to oneembodiment described herein.

FIG. 4 is a flowchart illustrating packetization of a SONET frame,according to one embodiment described herein.

FIG. 5 is a flowchart illustrating migration of a legacy synchronousoptical networking network to a packet network, according to oneembodiment described herein.

FIGS. 6A-D illustrate migration of a legacy synchronous opticalnetworking network to a packet network, according to one embodimentdescribed herein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

Embodiments described herein include a method for emulating a timedivision multiplexed (TDM) circuit using a packet switched network. Themethod includes installing a first packet node and a second packet nodein a communication network including a first TDM node and a second TDMnode, the first TDM node and the second TDM node forming a first span inthe TDM circuit. The method further includes disconnecting a first fiberconnection from the first TDM node and connecting the first fiberconnection to the first packet node. The method further includesdisconnecting a second fiber connection from the second TDM node andconnecting the second fiber connection to the second packet node. Themethod further includes emulating a portion of TDM data transmissionacross the first span using a packet connection formed by the firstpacket node and the second packet node. The TDM data transmissionincludes transport overhead data, and the packet connection emulates thetransport overhead data

Embodiments described herein further include a system. The systemincludes a first packet node and a second packet node communicativelycoupled to form a packet connection in a packet switched network. Thesystem further includes a first time division multiplexed (TDM) node anda second TDM node communicatively coupled to form a first span in a TDMcircuit. The system further includes a third TDM node and a fourth TDMnode communicatively coupled to form a second span in the TDM circuit.The packet connection is configured to emulate a portion of TDM datatransmission across the first span, the TDM data transmission includestransport overhead data, and the packet connection emulates thetransport overhead data, and

Embodiments described herein further include a packet node in acommunication network. The packet node includes a network connectionthat communicatively couples the packet node to a second packet node andforms a packet connection in the communication network. Thecommunication network includes a first time division multiplexed (TDM)node and a second TDM node communicatively coupled to form a first spanin a TDM circuit. The communication network further includes a third TDMnode and a fourth TDM node communicatively coupled to form a second spanin the TDM circuit. The packet connection is configured to emulate aportion of TDM data transmission across the first span, the TDM datatransmission includes transport overhead data, and the packet connectionemulates the transport overhead data. The second span in the TDM circuitis operable to transport data using TDM while the packet connection isconfigured to emulate the portion of TDM data transmission across thefirst span.

Example Embodiments

Synchronous optical networking (SONET) and synchronous digital hierarchy(SDH) are protocols for transferring multiple digital bit streamssynchronously over optical fiber using lasers or other light sources.SONET is often uses in the United States and Canada, while SDH is oftenused in the rest of the world. The disclosure herein will focus onSONET, but these techniques are equally applicable to SDH.

SONET typically includes dedicated data communication channels (DCCs)and other data (e.g., K1 and K2 bytes) as part of transport overheaddata (TOH). This TOH facilitates path protection, inband management, andother network management functions. These functions can be managed by anetwork management system (NMS). The TOH can facilitate an inbandmanagement connection between SONET devices and NMS systems. Further, insome circumstances, the TOH can be used to represent the physicaltopology of the SONET network to circuit routing logic for pathcomputation.

Therefore, when emulating a SONET network using a packet network, it isdesirable to emulate the TOH as well as the data being transported. Onetechnique to do this is to emulate the whole SONET interface. But thiscan be extremely bandwidth intensive and wasteful. For example,emulating a complete SONET interface could require approximately 11 Gbpsof packet bandwidth. This is inefficient, and rules out migrating aSONET network using 10 Gbps packet network links.

Circuit emulation over packet (CEP) technology allows a packet networkto emulate only paths (i.e., time slots) that are actually in use tocarry user data for the SONET time division multiplexed (TDM)connections and services. For example, CEP techniques allow theemulation of a particular synchronous transport signal (STS) within theSONET frame—for example, only the STS actually in use. Emulation of aparticular STS can be done with approximately 50 Mbps of packetbandwidth, making this much more efficient than emulating the wholeSONET interface. Further, CEP techniques can be used to emulateparticular virtual tributaries (VTs) within an STS payload. A given VTcan be emulated with as little as 2 Mbps of packet bandwidth, makingthis even more efficient.

But in traditional CEP techniques, the TOH data is lost. The TOH data isterminated by the packet system and not carried over to thecorresponding TDM node (e.g., an add-drop multiplexer (ADM)). One ormore techniques disclosed herein describe use of a circuit emulationconnection to carry the TOH and solve this problem. This allows for TOHtransparency, allowing the NMS to carry on with the various networkmanagement functions facilitated by the TOH data, with the bandwidthefficiency of CEP techniques.

FIG. 1A illustrates a legacy SONET network, according to one embodimentdescribed herein. A network 100 includes a SONET ring 110 that isconnected to a TDM network 120. The SONET ring 110 includes the SONETnodes 112, 114, and 116. In the illustrated embodiment, the SONET node112 is located in physical location 1, the SONET node 116 is located inphysical location 2, and the SONET node 114 is located between the SONETnodes 112 and 116. As discussed above, the illustrated disclosurefocuses on SONET and FIG. 1A illustrates a SONET ring 110, but thetechniques disclosed are equally applicable to an SDH ring.

The TDM network 120 includes the TDM circuits 122 and 124. In anembodiment, the TDM circuit 124 is located in physical location 3. In anembodiment, data can travel from the SONET node 112 to the TDM circuit124 via the path 128. Further, data can travel from the SONET node 112to the TDM circuit 122 via the path 126.

FIG. 1B illustrates a packet network emulating the legacy SONET networkof FIG. 1A, according to one embodiment described herein. In anembodiment, FIG. 1B corresponds to the end result after migration of theSONET network in FIG. 1A to a packet network. The network 100 includes apacket switched network 160 (e.g., an Ethernet network) and a packetswitched network 170. The packet network 160 includes the packet nodes162, 164, and 166. In an embodiment, the packet node 162 (e.g., anEthernet node) is located at physical location 1, the same physicallocation as the SONET node 112. In an embodiment, the packet node 166 islocated at physical location 2, the same physical location as the SONETnode 116. The packet node 164 is located at the same physical locationas the SONET node 114. Alternatively, the packet nodes 162, 164, and 166are located in different physical locations from the SONET nodes 112,114, and 116.

The packet network 170 includes emulated TDM circuits 172 and 174. In anembodiment, the emulated TDM circuit 172 is located at the same physicallocation as the TDM circuit 122, and is used to emulate the TDM circuit122. Further, in an embodiment, the emulated TDM circuit 174 is locatedat the same physical location as the TDM circuit 124 and is used toemulate the TDM circuit 124. In an embodiment, the packet network 160and the packet network 170 are used in place of the SONET ring 110 andthe TDM network 120. Data travels from the packet node 162 to theemulated TDM circuit 172 via the path 176, while data travels from thepacket node 162 to the emulated TDM circuit 174 via the path 178.

FIG. 2 illustrates a circuit emulation controller 200, according to oneembodiment described herein. As shown, the circuit emulation controller200 includes, without limitation, a central processing unit (CPU) 202, anetwork interface 206, a memory 210, and storage 270, each connected toa bus 208. In an embodiment, the circuit emulation controller 200 alsoincludes an Input/Output (I/O) device interface 204 for connecting toI/O devices 260. In an embodiment, the I/O devices 260 can be externalI/O devices (e.g., keyboard, display and mouse devices). Alternatively,the I/O devices 260 can be built in I/O devices (e.g., a touch screendisplay or touchpad). As another alternative, the circuit emulationcontroller 200 does not include its own I/O device interface 204 and isinstead managed remotely using the network interface 206.

The CPU 202 retrieves and executes programming instructions stored inthe memory 210 as well as stores and retrieves application data residingin the storage 270. The bus 208 is used to transmit programminginstructions and application data between the CPU 202, the I/O deviceinterface 204, the storage 270, the network interface 206, and thememory 210. The CPU 202 is included to be representative of a CPU,multiple CPUs, a single CPU having multiple processing cores, graphicsprocessing units (GPUs) having multiple execution paths, and the like.The memory 210 is generally included to be representative of electronicstorage of any suitable type(s), including random access memory ornon-volatile storage. The storage 270 may be a disk drive storagedevice. Although shown as a single unit, the storage 270 may be acombination of fixed or removable storage devices, such as fixed discdrives, removable memory cards, network attached storage (NAS), or astorage area-network (SAN).

Although the memory 210 is shown as a single entity, the memory 210 mayinclude one or more memory devices having blocks of memory associatedwith physical addresses, such as random access memory (RAM), read onlymemory (ROM), flash memory or other types of volatile and/ornon-volatile memory. The memory 210 generally includes program code forperforming various functions related to use of the circuit emulationcontroller 200. The program code is generally described as variousfunctional “applications” or “modules” within the memory 210, althoughalternate implementations may have different functions and/orcombinations of functions. Within the memory 210, the circuit emulationmodule 220 facilitates migration of a legacy synchronous opticalnetworking network to a packet network, as described in relation tosubsequent Figures. The circuit emulation module 220 further includes apacketizing module 222. The packetizing module 222 is generallyconfigured to facilitate packetization of a SONET frame, as described inrelation to FIGS. 3A-C, below.

In an embodiment, the circuit emulation controller 200 can beimplemented as part of an packet node in the packet network (e.g., oneor more of the packet nodes 162, 164, and 166 illustrated in FIG. 1B).Alternatively, the circuit emulation controller 200 can be implementedas a separate system in communication with the packet nodes. In oneembodiment, the circuit emulation controller 200 is a single system.Alternatively, the circuit emulation controller 200 is distributedacross multiple systems.

FIGS. 3A-C illustrate packetization of a SONET frame, according to oneembodiment described herein. FIG. 3A illustrates a SONET frame 300. TheSONET frame 300 includes a transport overhead 302 and an STS envelope304. In an embodiment, the transport overhead 302 includes the DCC andother TOH data (e.g., K1 and K2 bytes). In an embodiment, the STSenvelope 304 includes the user data for transmission.

FIG. 3B illustrates the SONET frame 300, with CEP packetization. In anembodiment, an STS 350 currently in use to carry user data ispacketized, as illustrated with the boundary 360. The STS 350 includesseveral VTs, including the VT 352. The VT 352 is packetized as part ofthe packetization of the STS 350. In an embodiment, this packetizationis done as part of standard CEP techniques.

FIG. 3C illustrates the SONET frame 300 with TOH packetization. Asillustrated by the boundary 362, in addition to the STS 350 thetransport overhead 302 is also packetized. In an embodiment, this isdone using the techniques described in relation to FIG. 4.Alternatively, other packetization techniques can be used.

FIG. 4 is a flowchart illustrating packetization of a SONET frame,according to one embodiment described herein. In an embodiment, thiscorresponds to the SONET frame illustrated in FIG. 3C. At block 402, apacketization module (e.g., the packetization module 222 within thecircuit emulation module 220 illustrated in FIG. 2) selects thetransport overhead bytes from a SONET frame (e.g., the transportoverhead 302 illustrated in FIGS. 3A-C). In an embodiment, thepacketization module selects all the transport overhead bytes in theSONET frame. In an alternative embodiment, the packetization moduleselects a subset of the transport overhead bytes. For example, thepacketization module can be configured to select only bytes that areused by the particular network being migrated. Alternatively, thepacketization module can identify transport overhead bytes actuallycarrying data and select those bytes.

At block 404, the packetization module includes the transport overheadbytes in the payload of a packet (e.g., a packet carrying data). Forexample, in a multiprotocol label switching (MPLS) network, thepacketization module includes the transport overhead bytes in thepayload of an MPLS packet. Further, the packetization module assigns asequence number to the transport overhead bytes. For example, thepacketization module can spread the transport overhead bytes acrossmultiple MPLS packets, and can assign the data in each packet a sequencenumber so that the data can be re-assembled when it is received.

In an embodiment, the packetization module includes the transportoverhead in packets that are not carrying the SONET envelope data. Inthis embodiment, the packetization module includes the transportoverhead in separate packets from the packets carrying the SONETenvelope data. For example, the circuit emulation controller (e.g., thecircuit emulation controller 200) can establish a pseudowire to carrypackets containing the transport overhead, and can transmit a dedicatedflow of packets (e.g., MPLS packets) carrying the transport overhead.Alternatively, the packetization module includes the transport overheadin packets carrying the SONET envelope data, or in any other suitablepackets.

At block 406, the circuit emulation controller (e.g., the circuitemulation controller 200) transports the packet including the transportoverhead bytes over the packet network to a destination that is incommunication with the remaining TDM network. For example, in an MPLSnetwork, the MPLS packets including the transport overhead bytes can betransmitted across the network using typical MPLS techniques. At block408, the recipient node receives the packet and places it in a buffer(e.g., a de-jitter buffer or any other suitable buffer). As discussedabove, in an embodiment the packet carrying the transport overhead bytesdoes not carry the SONET envelope data.

At block 410, a packetization module in communication with the recipientnode extracts the transport overhead bytes from the packet payloads andre-assembles the transport overhead bytes. In an embodiment, thereceived bytes further include a sequence number (as discussed above),and the packetization module uses the sequence number to re-assemble thetransport overhead bytes. At block 412, the circuit emulation controllerforwards the transport overhead bytes across the TDM line. In anembodiment, the recipient node is communicatively coupled to the TDMcircuit, and transports the TDM overhead bytes across the existing TDMline (e.g., paths 126 or 128 illustrated in FIG. 1A).

FIG. 5 is a flowchart illustrating migration of a legacy synchronousoptical networking network to a packet network, according to oneembodiment described herein. FIGS. 6A-D illustrate migration of a legacysynchronous optical networking network to a packet network, according toone embodiment described herein. In an embodiment, the flowchart of FIG.5 corresponds with the network configurations illustrated in FIGS. 6A-D.Consequently, FIGS. 5 and 6A-D will be discussed together.

At block 502, a network administrator installs packet nodes adjacent toSONET nodes in the legacy network, so that the fiber connections to theSONET nodes can be transferred to the packet nodes. These packet nodescorrespond to the SONET nodes. For example, as illustrated in FIG. 6A, anetwork 600 includes several SONET nodes 612, 614, and 618. At block502, packet nodes are installed adjacent to these nodes and correspondto the SONET nodes 612, 614, and 618. For example, a packet node 662 isinstalled adjacent to the SONET node 612 and the packet node 662corresponds to the SONET node 612, a packet node 664 is installedadjacent to the SONET node 614 and corresponds to the SONET node 614,and a packet node 668 is installed adjacent to the SONET node 618 andcorresponds to the SONET node 618. In an embodiment, the networkadministrator also installs packet nodes adjacent to the TDM circuits(e.g., the TDM circuits 622 and 624). In an embodiment, the packet nodesare located physically adjacent to the corresponding SONET nodes (e.g.,in the same physical location), and are directly connected. In anembodiment, a packet node is installed adjacent to a SONET node so thata fiber connection to the SONET node (e.g., a fiber connection to anadd-drop multiplexer) can be transferred to the packet node, instead.Transferring the connection includes disconnecting the fiber connectionfrom the SONET node and connecting the fiber connection to the packetnode, instead. Alternatively, the packet nodes are located in adifferent physical location from the corresponding SONET nodes, but areconnected to the SONET nodes, either directly or indirectly.

At block 504, a network administrator prepares the ring for migration.In an embodiment, the network administrator moves fibers from the SONETnodes (e.g., the add-drop multiplexers (ADMs)) in the SONET ring (e.g.,the SONET ring 610) to the packet nodes. This is illustrated further inFIG. 6A, in which the packet network 660 is connected to emulate trafficthrough the SONET ring 610. The network 600 includes the SONET ring 610that is connected to a TDM network 620, and the packet switched networks660 (e.g., an Ethernet network) and 670. The packet nodes 662, 664, and668 are connected using the fibers that were previously used to connectthe SONET ADMs. While the fiber is moved the network administrator canavoid any impact on traffic by leveraging existing link protectiontechniques, for example unidirectional path-switched rings (UPSR) in aSONET network (or subnetwork connection protection (SNCP) in an SDHnetwork). These link protection techniques provide for multipleredundant paths for traffic in the network, so that moving fibers one byone will not impact traffic.

In an embodiment, a circuit emulation controller (e.g., the circuitemulation controller 200 illustrated in FIG. 2) establishes circuitemulation connections between the packet nodes. For example, the circuitemulation controller establishes an emulated path between the packetnodes 662, 664, and 668, as illustrated in FIG. 6A. This path emulatesboth the TDM data and the transport overhead, as discussed above inrelation to FIG. 4.

In an embodiment, the packet network 660 is an MPLS network, and labeldistribution protocol (LDP) techniques are used to establish pseudowireconnections between the nodes in the packet network. In an embodiment, apseudowire is a virtual connection between two packet devices used toemulate the SONET TDM connection. A given TDM timeslot or transportoverhead component is selected to be part of a pseudowire connection. Inan embodiment, the TDM timeslot or transport overhead component isselected by a system administrator, automatically, or using any othersuitable technique. This selection is done at both ends of theconnection (e.g., at the packet node 662 and the packet node 668 for apseudowire connection between these nodes). Using LDP techniques, eachnode at the end of the pseudowire is provided with a virtual connection(VC) label to use with packets transported over the pseudo-wire betweenthe nodes. That label is then used for routing the packet over the MPLSpacket network. Further, the label can be used when de-packetizing thetraffic to re-create the SONET frame and ensure the data is transportedover the appropriate TDM connection.

At block 506, a network administrator identifies the next TDM circuitfor migration. For example, as illustrated in FIG. 6A, the next TDMcircuit for migration is TDM circuit 624. At block 508, a networkadministrator migrates the identified TDM circuit path. As illustratedin FIG. 6A, the legacy network includes a path 628 from the SONET node612 to the TDM circuit 624. The circuit emulation controller migratesthis path to the packet network. As illustrated in FIG. 6B, new path 678carries traffic from the packet node 662 to the emulated TDM circuit674. This new path 678 in FIG. 6B replaces the legacy path 628 in FIG.6A.

In an embodiment, the network administrator migrates the path 628 to thepath 678 by moving customer facing TDM ports from the SONET ADM (e.g.,the SONET node 612) to the packet nodes (e.g., the packet node 662). Thenetwork administrator then provisions a circuit emulation connectionbetween the packet node (e.g., the packet node 662) and the emulated TDMcircuit (e.g., the emulated TDM circuit 674). This can be done in asimilar fashion to the techniques for migrating the SONET ring,discussed above in relation to block 504, and will not be repeated here.In an embodiment, after a TDM circuit is migrated (e.g., the TDM circuit624), the respective circuit emulation connection associated with thepacket network 660 can be removed. For example, as illustrated in FIG.6B, the connection between the packet nodes 662, 664, and 668 can beremoved.

Returning to FIG. 5, at block 510 the network administrator determineswhether all TDM circuits have been migrated. In the network illustratedin FIG. 6B, they have not, so the flow returns to block 506. The networkadministrator identifies the next TDM circuit for migration. In thiscase, as illustrated in FIG. 6B, the TDM circuit 622 is next formigration.

At block 508, during the second pass, the network administrator migratesthis second TDM circuit. As illustrated in FIG. 6B, the partiallymigrated network includes a path 626 from the SONET node 612 to the TDMcircuit 622. The circuit emulation controller migrates this path to thepacket network. As illustrated in FIG. 6C, new path 676 carries trafficfrom the packet node 662 to the emulated TDM circuit 672. This new path676 in FIG. 6C replaces the legacy path 626 in FIG. 6B. This migrationcan be done using the techniques described above for migrating the path628 to the path 678.

At block 510, the network administrator again determines whether all TDMcircuits have been migrated. In the network illustrated in FIG. 6C, theyhave, and so the flow proceeds to block 512. At block 512, the networkadministrator finalizes the migration. In an embodiment, the remainingcircuit emulation for the SONET ring (e.g., the connections in thepacket network 660 emulating the SONET ring 610) is removed. Further,the SONET ring is decommissioned, because all traffic now flows throughthe packet network. This is illustrated in FIG. 6D, in which the SONETring 610 has been decommissioned and the TDM circuits 622 and 624 havebeen replaced with the emulated TDM circuits 672 and 674.

One or more of the techniques disclosed above have many advantages overcurrent solutions. As one example, in at least some embodiments thecontrol plane remains in place throughout migration of a SONET networkto a packet network, because the TOH is transparent and emulated by thepacket network during migration. As another example, in at least someembodiments each TDM circuit can be migrated individually, withoutmigrating the complete legacy network. A network administrator can waitdays, weeks, months, or even years, between migrating different TDMcircuits. This is because the circuit emulation includes transparent TOHdata, in addition to the user data, keeping the network fully functionalduring migration.

Further, a network administrator might choose to migrate only a portionof the legacy TDM network. The network administrator can migrate anysubset of the TDM circuits to packet connections, while leaving theremaining TDM circuits in place. This allows for both packet-switchedand SONET TDM connections over the network, simultaneously. As anotherexample, at least some of the techniques disclosed herein maintain theSONET topology during migration. This allows networks that make use ofthis topology for routing to seamlessly continue, without interruption.For example, a network that relies on network topology for routing willbe able to continue routing, while part of the network is emulated usingpacket a packet switched network.

In the preceding, reference is made to embodiments presented in thisdisclosure. However, the scope of the present disclosure is not limitedto specific described embodiments. Instead, any combination of thedescribed features and elements, whether related to differentembodiments or not, is contemplated to implement and practicecontemplated embodiments. Furthermore, although embodiments disclosedherein may achieve advantages over other possible solutions or over theprior art, whether or not a particular advantage is achieved by a givenembodiment is not limiting of the scope of the present disclosure. Thus,the preceding aspects, features, embodiments and advantages are merelyillustrative and are not considered elements or limitations of theappended claims except where explicitly recited in a claim(s).

As will be appreciated by one skilled in the art, the embodimentsdisclosed herein may be embodied as a system, method or computer programproduct. Accordingly, aspects may take the form of an entirely hardwareembodiment, an entirely software embodiment (including firmware,resident software, micro-code, etc.) or an embodiment combining softwareand hardware aspects that may all generally be referred to herein as a“circuit,” “module” or “system.” Furthermore, aspects may take the formof a computer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium is any tangible medium that can contain, or store a program foruse by or in connection with an instruction execution system, apparatusor device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent disclosure may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present disclosure are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodimentspresented in this disclosure. It will be understood that each block ofthe flowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

Embodiments of the invention may be provided to end users through acloud computing infrastructure. Cloud computing generally refers to theprovision of scalable computing resources as a service over a network.More formally, cloud computing may be defined as a computing capabilitythat provides an abstraction between the computing resource and itsunderlying technical architecture (e.g., servers, storage, networks),enabling convenient, on-demand network access to a shared pool ofconfigurable computing resources that can be rapidly provisioned andreleased with minimal management effort or service provider interaction.Thus, cloud computing allows a user to access virtual computingresources (e.g., storage, data, applications, and even completevirtualized computing systems) in “the cloud,” without regard for theunderlying physical systems (or locations of those systems) used toprovide the computing resources.

Typically, cloud computing resources are provided to a user on apay-per-use basis, where users are charged only for the computingresources actually used (e.g. an amount of storage space consumed by auser or a number of virtualized systems instantiated by the user). Auser can access any of the resources that reside in the cloud at anytime, and from anywhere across the Internet. In context of the presentinvention, a user may access applications (e.g., the circuit emulationmodule 220) or related data available in the cloud. For example, thecircuit emulation module 220 could execute on a computing system in thecloud and facilitate emulation of the TDM network by the packet network.Doing so allows a user to access this information from any computingsystem attached to a network connected to the cloud (e.g., theInternet).

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality and operation of possible implementations ofsystems, methods and computer program products according to variousembodiments. In this regard, each block in the flowchart or blockdiagrams may represent a module, segment or portion of code, whichcomprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

We claim:
 1. A method for emulating a time division multiplexed (TDM)circuit using a packet switched network, comprising: installing a firstpacket node and a second packet node in a communication networkcomprising a first TDM node and a second TDM node, the first TDM nodeand the second TDM node forming a first span in the TDM circuit;disconnecting a first fiber connection from the first TDM node andconnecting the first fiber connection to the first packet node;disconnecting a second fiber connection from the second TDM node andconnecting the second fiber connection to the second packet node; andemulating a portion of TDM data transmission across the first span usinga packet connection formed by the first packet node and the secondpacket node, wherein the TDM data transmission comprises transportoverhead data, and wherein the packet connection emulates the transportoverhead data.
 2. The method of claim 1, wherein emulating the portionof the TDM data transmission across the first span using the packetconnection further comprises configuring the packet connection to:select a transport overhead byte from a frame in the TDM datatransmission; include the transport overhead byte in a packet of thepacket connection; transmit the packet over the packet connection;receive the packet over the packet connection; extract the transportoverhead byte from the received packet; and forward the transportoverhead byte across the TDM circuit.
 3. The method of claim 2, whereinthe packet connection comprises a multiprotocol label switching (MPLS)connection, wherein the transport overhead byte is carried in an MPLSpacket, and wherein a sequence number related to the transport overheadbyte is added to the MPLS packet.
 4. The method of claim 1, wherein theTDM circuit comprises at least one of a synchronous optical networking(SONET) or a synchronous digital hierarchy (SDH) circuit.
 5. The methodof claim 1, wherein the packet connection comprises a circuit emulationover packet (CEP) connection.
 6. The method of claim 1, furthercomprising: installing a third packet node and a fourth packet node inthe communication network, wherein the communication network furthercomprises a third TDM node and a fourth TDM node forming a second spanin the TDM circuit; disconnecting a third fiber connection from thethird TDM node and connecting the third fiber connection to the thirdpacket node; disconnecting a fourth fiber connection from the fourth TDMnode and connecting the fourth fiber connection to the fourth packetnode; and emulating a portion of a second TDM data transmission acrossthe second span using a second packet connection, wherein the second TDMdata transmission comprises second transport overhead data, and whereinthe second packet connection emulates the second transport overheaddata.
 7. The method of claim 6, wherein the first span in the TDMcircuit comprises a first span of a SONET or SDH ring and the secondspan in the TDM circuit comprises a second span of the SONET or SDHring.
 8. The method of claim 7, further comprising: determining that allspans in the ring have been emulated using at least one packetconnection, and in response emulating TDM data transmission across athird span using a third packet connection, wherein the third span isoutside of the SONET or SDH ring.
 9. The method of claim 1, wherein: theTDM circuit comprises a second span, data transmission across the secondspan uses TDM, and data transmission across the first span is emulatedusing the packet connection.
 10. A system comprising: a first packetnode and a second packet node communicatively coupled to form a packetconnection in a packet switched network; a first time divisionmultiplexed (TDM) node and a second TDM node communicatively coupled toform a first span in a TDM circuit; and a third TDM node and a fourthTDM node communicatively coupled to form a second span in the TDMcircuit, wherein the packet connection is configured to emulate aportion of TDM data transmission across the first span, wherein the TDMdata transmission comprises transport overhead data, and wherein thepacket connection emulates the transport overhead data, and the secondspan is configured to transmit data using TDM while the packetconnection is configured to emulate the portion of TDM data transmissionacross the first span.
 11. The system of claim 10, wherein the packetconnection is configured to emulate the portion of the TDM datatransmission across the first span by: selecting a transport overheadbyte from a frame in the TDM data transmission; including the transportoverhead byte in a packet of the packet connection; transmitting thepacket over the packet connection; receiving the packet over the packetconnection; extracting the transport overhead byte from the receivedpacket; and forwarding the transport overhead byte across the TDMcircuit.
 12. The system of claim 11, wherein the packet connectioncomprises a multiprotocol label switching (MPLS) connection, wherein thetransport overhead byte is carried in an MPLS packet, and wherein asequence number related to the transport overhead byte is added to theMPLS packet.
 13. The system of claim 10, wherein the packet connectioncomprises a circuit emulation over packet (CEP) connection.
 14. Thesystem of claim 10, wherein the first span in the TDM circuit comprisesa first span of a synchronous optical networking (SONET) or asynchronous digital hierarchy (SDH) ring and the second span in the TDMcircuit comprises a second span of the SONET or SDH ring.
 15. A packetnode in a communication network, comprising: a network connection thatcommunicatively couples the packet node to a second packet node andforms a packet connection in the communication network, wherein thecommunication network further comprises: a first time divisionmultiplexed (TDM) node and a second TDM node communicatively coupled toform a first span in a TDM circuit; and a third TDM node and a fourthTDM node communicatively coupled to form a second span in the TDMcircuit, and wherein the packet connection is configured to emulate aportion of TDM data transmission across the first span, wherein the TDMdata transmission comprises transport overhead data, and wherein thepacket connection emulates the transport overhead data, and the secondspan in the TDM circuit is operable to transport data using TDM whilethe packet connection is configured to emulate the portion of TDM datatransmission across the first span.
 16. The packet node of claim 15,wherein the packet node is configured to emulate the portion of the TDMdata transmission across the first span by: selecting a transportoverhead byte from a frame in the TDM data transmission; including thetransport overhead byte in a first packet of the packet connection;transmitting the first packet over the packet connection; receiving asecond packet over the packet connection; extracting the transportoverhead byte from the received second packet; and forwarding thetransport overhead byte across the TDM circuit.
 17. The packet node ofclaim 16, wherein the second packet is received from the second packetnode and wherein extracting the transport overhead byte from thereceived second packet comprises storing the received second packet in ade-jitter buffer.
 18. The packet node of claim 16, wherein the packetconnection comprises a multiprotocol label switching (MPLS)connection,wherein the transport overhead byte is carried in an MPLS packet, andwherein a sequence number related to the transport overhead byte isadded to the MPLS packet.
 19. The packet node of claim 15, wherein thepacket connection comprises a circuit emulation over packet (CEP)connection.
 20. The packet node of claim 15, wherein the first span inthe TDM circuit comprises a first span of a synchronous opticalnetworking (SONET) or a synchronous digital hierarchy (SDH) ring and thesecond span in the TDM circuit comprises a second span of the SONET orSDH ring.